1. Field of the Invention
This invention relates to molded products of polypropylene which have excellent physical properties such as stiffness, heat resistance and oil resistance, and, particularly, to stretched films and molded products of polypropylene for electrical insulation.
2. Description of the Prior Art
Polypropylenes are thermoplastic resins which generally have good mechanical strength, heat resistance, moisture resistance, etc. They are molded, alone or in a composition with various other materials, into various articles such as structural materials, films, containers, fibers and so on by molding procedures such as injection molding, extrusion molding, etc. These molded articles find wide application in automobile parts, packaging materials, electrical components and equipment, carpets, etc.
However, polypropylenes conventionally used do not necessarily have satisfactory mechanical strength, heat resistance and so on in certain applications. For instance, biaxially stretched polypropylene films have inferior stiffness to cellophane and polyester films although they are packaging materials having excellent optical properties (such as transparency, luster, etc.) and moisture resistance. As a result polypropylenes are not suitable for use in automatic packaging, especially overlap packaging and twist packaging, leading to a great limitation on their use. Moreover, even in cases where the polypropylene films can be made much thinner from the standpoint of moisture resistance and other properties, it is inevitably necessary to increase their film thickness to obtain stiffness. This is not only uneconomical but is also an obstacle to the miniaturization of electrical components such as dry condensers wherein a polypropylene film is used.
Because of their excellent dielectric properties and high electrical breakdown strength, polypropylenes are now commonly used as insulating materials for electrical equipment such as extra high voltage cables and condensers. However, in using polypropylenes as insulation materials in electric equipment, problems occur in that polypropylene films swell in the insulation oils used in the electrical equipment (such as mineral oil and hydrocarbon insulation oil, e.g., alkylbenzene, alkylnaphthalene, diaryl ethane and the like), thereby increasing the film thickness. The increase in the thickness of the film due to the swelling leads to a quite serious problem as hereinafter explained. Further, the polypropylene dissolves in the insulation oil thereby increasing the viscosity of the insulation oil.
In the case of a polypropylene laminated paper in which polypropylene is laminated on a kraft paper, the increase in the thickness of the polypropylene film leads first to a reduction in the mobility of the insulation oil contained in an insulation layer. Mobility is needed to compensate for swell and contraction of the insulation oil with changes in the temperature of the electrical equipment.
In hitherto used electrical equipment in which a kraft paper is used as an insulation, the insulation oil can pass through the thickness of the kraft paper because the kraft paper is porous. In the polypropylene laminated paper, however, the insulation oil cannot pass. Therefore, the insulation oil must flow by passing through the spaces or pores on the surface of the kraft paper or in the inner portion thereof. Thus, the mobility of the insulation oil is inherently inferior even when there is not an increase in the thickness of the polypropylene layer. In this case, however, the use of a kraft paper having a relatively low density ensures oil mobility to the extent that it is usually required in conventional electrical equipment. Nevertheless, if the polypropylene layer swells and increases in thickness, the kraft paper is compressed thereby decreasing the spaces or pores, and ultimately the necessary fluidity of the insulation oil cannot be maintained.
Second, the layers of tape in contact with each other around a cable are pressed more firmly through the compression of the kraft paper upon swelling of the polypropylene, and the ability of the tapes to slide becomes difficult. This leads to the problem that the application of bending onto the cables, etc., in the installation thereof produces wrinkles on the tapes. These phenomena become more marked as the extent to which the polypropylene layer swells increases and the increase in the film thickness increases, and as the ratio of the thickness of the polypropylene layer to the total thickness of the polypropylene laminated paper increases. Thus, if the polypropylene layer swells to a greater extent, the ratio of the polypropylene layer to the total of the polypropylene laminated paper must be made smaller.
From the standpoint of electrical properties, however, it is preferred that the ratio of the polypropylene layer to the total thickness of the laminate, which has excellent dielectric properties, etc., is high. Thus, an insulation material consisting of a polypropylene film is most preferred. For these reasons, in obtaining electrical components of excellent characteristics, it is important to minimize the swelling property of the polypropylene layer.
Polypropylenes having a relatively low molecular weight are dissolved in the insulation oil from the polypropylene layer and increase the viscosity of the insulation oil, which deteriorates the fluidity and mobility of the oil in the insulation layer. Therefore, it is also important to minimize the increase in the vicosity of the insulation oil due to the polypropylene layer.
In order to solve the above-described problems, an attempt to improve physical properties such as stiffness of the molded products of polypropylene has been made. One of the procedures proposed is to increase the isotactic index (I.I). In accordance with this procedure, those polypropylenes produced by decreasing the amount of the atactic portion produced during polymerization of polypropylene or by removing the atactic portion by washing with a solvent are employed. This procedure, however, is directed only to increasing the isotactic index and the stiffness and the degree of swelling are insufficiently improved. Furthermore, the viscosity of the insulation oil is markedly increased. Particularly at relatively high temperatures no improvement is observed.
Another procedure is disclosed in Japanese Patent Publication No. 14062/64, in which a nucleating agent is added to polypropylene. Another procedure is disclosed in Japanese Patent Publication No. 11656/72, in which a modifying agent such as a styrene polymer and the like is added to polypropylene. However, in accordance with the procedure disclosed in Japanese Patent Publication No. 14062/64, the optical and electrical properties of the stretched film are deteriorated. In accordance with the procedure disclosed in Japanese Patent Publication No. 11656/72, a large amount of the modifying agent is added which deteriorates the heat resistance, oil resistance and electrical properties of the polypropylene, and increases costs. It can be readily understood that with these procedures it is quite difficult to maintain the inherent characteristics of polypropylene because the stiffness of polypropylene is improved by the addition of foreign substances.
As another procedure for obtaining stretched films of high stiffness, it has been proposed to increase the stretching ratio. In this procedure, however, the stretching is carried out under severe conditions. Therefore, troubles such as film splitting during stretching are likely to take place and the productivity is poor.
One procedure of controlling the swelling property of polypropylene is by subjecting a laminated paper to heat treatment in a vacuum at a temperature below the melting point of polypropylene. In accordance with this procedure, however, the swelling property is insufficiently improved, and the viscosity of the insulation oil is thus markedly increased. Furthermore, since the heat treatment is carried out under specific conditions, the cost increases and this procedure is thus disadvantageous from the economic standpoint.
Based on the assumption that it is not possible to eliminate all swelling and that some increase in thickness resulting from swelling in commercially available electrical equipment in which polypropylene is employed must occur, Japanese Patent Application (OPI) No. 96083/78 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a procedure in which a plurality of polypropylene layers are formed and a layer capable of absorbing the increase in thickness is provided between the polypropylene layers to thereby reduce the apparent increase in the thickness of the total laminated paper. Although this procedure is very effective in decreasing the expansion due to swelling it has the disadvantage that the structure of the laminated paper is made complicated, and the production is thus more expensive.